Fiber reinforced plastics (referred to as “FRP” hereinafter) composed of reinforcing fibers having high strength and high elasticity modulus, such as carbon fibers and the like, have excellent mechanical properties, and are thus versatile as structural materials for aircraft.
Although FRP has excellent mechanical properties in the fiber orientation direction, the mechanical properties in a direction deviated from the fiber axis rapidly deteriorate, i.e., the mechanical properties have great anisotropy. Therefore, in many cases in which FRP is used as a structural material for aircraft, a plurality of thin prepreg layers are laminated so that the fiber axes of adjacent layers deviate at about 30° to 60°, i.e., cross lamination is performed, to cause quasi-isotropy in the mechanical properties in the planar direction of FRP.
However, it is known that when an impact is applied to such a FRP plate in the thickness direction, the impact causes cracks between the respective layers of FRP because the layers have great anisotropy in mechanical properties, thereby causing delamination and significantly deteriorating the compression strength of the FRP plate receiving the impact.
As a countermeasure against this, for example, thermoplastic particles are adhered to a surface of prepreg to be arranged between the layers of the formed laminate so that the propagation energy of cracks due to impact force is absorbed by breakage of the particles, decreasing the area of delamination. This countermeasure significantly improves the residual compression strength of the FRP plate receiving the impact, which allows FRP to be used as a primary structural material for large civil aircraft.
However, this method increases the production cost of a FRP structural material due to the following causes:
A. The production cost of the thermoplastic particles having a uniform particle diameter is high because of the small particle diameter.
B. Since the particles are uniformly adhered to the resin surface of the prepreg, the working speed of the prepreg is decreased, or another new step is required for forming a resin film in which the particles are dispersed in a matrix resin in the B-stage state.
C. The particles enter the prepreg or the FRP layers after the resin of the prepreg is cured according to the production and molding conditions of the prepreg. This makes precisely arranging the predetermined particles between the layers difficult.
D. In autoclave molding using the prepreg, the use of the prepreg having tucks requires deaeration between the prepreg layers during lamination, and a plurality of thin prepreg layers must typically be laminated together in order to obtain a structural material having a predetermined thickness. This is a labor-intensive process.
Due to the declining cost of crude oil, aircraft makers are less inclined to purchase expensive light-weight materials. Accordingly, a way of reducing the production costs of FRP structural materials is desired.
For example, a resin transfer molding (RTM) method, in which the mold cavity is filled with a laminate of a fiber reinforcing material, and then a resin is injected has recently attracted attention as a low-cost molding method. However, this method cannot precisely arrange the thermoplastic particles between the layers of the laminate, and forming a high-toughness FRP having excellent impact resistance is difficult without an improvement in the resin. In addition, when the fiber reinforcing materials are simply laminated, the materials in the layers are deviated from each other making it difficult to handle and disturb the fiber orientation, and thus FRP having predetermined mechanical properties cannot be easily obtained.